The Las Vegas concert massacre has been confirmed as the deadliest mass shooting in modern US history

Dozens of patrol vehicles descended on the Strip last night after authorities received reports of an active gunman near a music festival.

Concert-goers reported seeing muzzle flashes from the upper floors of the Mandalay Bay across Las Vegas Boulevard, and the sound of what they described as automatic gunfire.

Kodiak Yazzie, 36, said he and his girlfriend were watching country star Jason Aldean's performance when he heard what sounded like fireworks. The music stopped temporarily and then started up again before another round of pops sent the performers ducking for cover and fleeing the stage.

Thousands fled as bursts of gunfire could be heard for more than five minutes, Mr Yazzie said.

Witnesses said they saw multiple victims and dozens of ambulances near the concert venue. Some attendees later huddled in the basement of the nearby Tropicana hotel-casino.

Police shut down the usually busy Las Vegas Boulevard and authorities across the state and federal ranks converged on the scene as dozens of ambulances ferried those struck by gunfire.

Nearby Interstate 15 and flights at McCarran International Airport were briefly closed.

Hospital emergency rooms were jammed with victims delivered by ambulance. Others loaded the wounded into their cars and drove them to hospitals.

Among those killed were two off-duty police officers who were attending the concert. Two on-duty officers were wounded, including one who underwent surgery and was upgraded to stable condition, police said.